Basically a possessive quantifier matches as much as it can and never gives any back. Thus it can be used to control backtracking. The syntax is similar to non-greedy matching, except instead of using a '?' as the modifier the '+' is used. Thus ?+, *+, ++, {min,max}+ are now legal quantifiers.

They often can be used instead of greedy matching...
Using it, the engine runs faster.
E.g \w+ \w+ With possessive quantifiers: \w++ \w++